Electronic devices, such as servers, include numerous electronic components that are powered by a common power supply. Servers generate an enormous amount of heat due to the operation of internal electronic components such as controllers, processors, and memory. Overheating from the inefficient removal of such heat has the potential to shut down or impede the operation of such devices. Thus, servers are designed to rely on air flow through the interior of the device to carry away heat generated from electronic components. Servers often include various heat sinks that are attached to the electronic components, such as processing units. Heat sinks are typically composed of thermally conductive material. Heat sinks absorb the heat from the electronic components, thus transferring the heat away from the components. The heat from heat sinks must be vented away from the server. Air flow to vent away such heat is often generated by a fan system. The generated air flow thus carries collected heat away from the components and the heat sink. Thus, air flow can pass through hot electric components in the device without any reverse air flow because the internal layout effectively channels the air flow.
A typical fan system will include multiple fans. Since fan noise increases exponentially with fan rotation speed, reducing rotations per minute (RPM) by a small amount potentially results in a large reduction in fan noise. One technique of modulating fan power is using a pulse width modulation control signal. Pulse width modulation turns the power supply to fan on and off at a fixed frequency. Duty-cycle adjustments may be made to control the speed of the fan. The larger the duty cycle, the faster the fan spins. A proper frequency must be selected since the fan's speed will noticeably oscillate within a PWM cycle if the signal frequency is too slow.
A large, but necessary component for a server is a power supply unit that converts AC power to DC power to power the electronic components. Failure of a power supply unit will result in the failure of the server. Thus, many server designs include multiple power supply units to provide a backup in case of failure of the primary power supply unit. A power supply unit generates heat from the process of converting input power, often in the form of AC voltage, via a transformer and rectifier, to direct current (DC) voltage output. A power supply unit may output the DC voltage, when connected to other components, or when controlled to output DC voltage. The conversion from AC to DC voltage generates heat. Generally, power supply units include an internal cooling fan to ensure the power supply unit does not overheat. Because such an active cooling fan is embedded in a power supply and therefore influences air flow, the location of the power supply unit inside a server is critical for air flow design. For most device designs, the power supply units and system fan wall will be placed in line between the back of the device and the front of the device. This type of in series placement avoids air circulation issues when one power supply unit fails.
FIG. 1A is a diagram of a prior art server 10 that includes various components 12. A fan wall 14 is located in series with two power supply units 16 and 18. The components 12 are cooled by the fan wall 14. As shown in FIG. 1A, this configuration places the power supply units 16 and 18 between the fan wall 14 and the components 12. The fan wall 14 in this example includes separate fans.
FIG. 1B is a diagram of another prior art server 30 that includes various components 32. Similar to the server 10, a fan wall 34 is located in series with two power supply units 36 and 38. The components 32 are cooled by the fan wall 34. As shown in FIG. 1B, this configuration places the fan wall 34 between the power supply units 36 and 38, and the components 32. In both of the servers 10 and 30 in FIGS. 1A and 1B, the in-line placement of the fan wall with the power supply unit generates air flow through the respective components and power supplies as shown in arrows 40. The placement of the power supplies in series with the fan wall means that no reverse air flow occurs through either power supply unit when one of the power supply units fails. While in-line placement of the fan wall with the power supply unit prevents reverse air flow, the arrangement of the components in the server layout is constrained by the in line placement.
In order to maximize air flow for some component placement considerations, certain server designs have the power supply unit and the system fans placed in parallel. FIG. 2 shows such a prior art server 60 that includes various electronic components 62. In this example, the server 60 includes two system fans 70 and 72 that are located in parallel with two power supply units 80 and 82. The fans 70 and 72 generate air flow through the device 60 as shown in by arrows 84. Changes in the air flow circulation generated by the fans 70 and 72 occur when one of the power supply units 80 or 82 fails.
In such designs, failure of a power supply unit such as the power supply unit 82, will cause reverse air flow, as shown by an arrow 86. The lack of air flow around the power supply unit 82 creates a low pressure area because the fans 70 and 72 continue to create air flow. The reverse air flow is generated mainly from a high pressure area backward to a low pressure area around the non-functioning power supply unit 82. The low pressure area creates reverse air flow as shown by the arrows 86.
When this kind of reverse air flow occurs, the internal fan in a power supply unit may fail to start normal rotation when the power supply unit is activated. For example, the fan of a power supply unit may fail due to reverse air flow when the power supply either wakes up from being in a cold redundant state or is newly installed. The reversed speed of the fan in the power supply unit depends on the duty cycle of the PWM signal to the system fans 70 and 72. When system fan duty cycle of the system fans 70 and 72 is high, such as during normal operation, air flow 84 is generated. However, reverse air flow 86 is also generated, and thus it is difficult for the internal fan in the power supply unit to start rotation normally. Typically, a reduction in system fan duty cycle for a short period of time allows the power supply unit fan to start. Such a reduction in fan duty cycle is controlled by software for a controller, such as a chassis management controller.
There are two situations when the internal fan on the power supply unit starts up. One situation is when the AC power is turned on to the power supply unit, and the other situation is when the DC power output is activated. In one example, the AC power being turned on may be detected by a PSU present pin. The DC power is activated when the power supply unit wakes up from cold redundant mode, such as when a power supply unit is brought on line when the primary power supply unit fails. In such instances, the activated power supply unit may not function correctly because the internal fan will not be rotating properly due to the reverse air flow.
Thus, there is a need for a system that ensures the proper start of a cold redundant power supply unit. There is another need for a system that uses both hardware and software to control system fan power to allow for the start of a cold redundant power supply unit. There is another need for a method and system that allows for an internal fan of a power supply unit to properly function when power is output by the power supply unit.